Purifying solutions for use in the electrowinning of chromium



1958 c. CAROSELLA 2,849,354

M. PURIFYING SOLUTIONS FOR USE IN THE ELECTROWINNING OF CHROMIUM Filed June 28, 1955 Make-up Make-up H2504 (NH so Crystals Iron-Chromium Alloy Reduced Anolyte--- Precipitate-- 1 Minor portion 15 Liqu" g m Residue Ammoniabearing Chrome Solution Impurities Basic Chrome Sulfate Precipitate (lontaining Pb 13 Elecl'rol l'lc Cell Cal'holyte Overf low Molasses 14,

Chromium Meral Anolyte Anol l'e Reductor 1N VEN TOR.

MICHAEL (l CAROSELLA fi ad. K 073 ATTORNE United. States Patent PURIFYING SOLUTIONS FOR USE IN THE ELECTROWINNING OF CHROMIUM Michael C. Carosella, Niagara Falls, N. Y., assignor to Union Carbide Corporation, a corporation of New York Application June 28, 1955, Serial No. 518,482

2 Claims. (Cl. 204-105) The present invention relates to a method for removing lead impurities from solutions employed in the electrowinning of chromium and, more particularly, to such a method for removing lead impurities from chromiumcontaining sulphate solutions employed as cell feed in the electrowinning of chromium.

Lead-alloy anodes and other lead-containing materials, such as agitator equipment, coil heaters, lead fittings, and the like, are widely employed in apparatus for the electrowinning of chromium. Such materials serve to contaminate the electrolyte and result in lead contamination of the deposited chromium metal. When electrolytically deposited chromium contaminated with lead is employed in high-chromium alloys, cracking invariably occurs during hot rolling of the alloy. In fact, so critical is the presence of lead impurity in the electrolytic chromium, that the manufacturers of high-chromium alloys tolerate only 0.003 percent maximum lead. In addition, the presence of lead impurity lowers the current efiiciency of the electrowinning operation considerably.

Accordingly, it is the prime object of the present invention to provide a method for the virtual elimination of lead contamination in solutions employed in the electrowinning of chromium.

Other aims and advantages of the invention will be apparent from the following description and appended claims.

In accordance with the present invention, chromiumcontaining sulphate solutions employed as cell feed in the electrowinning of chromium are adjusted in pH to a point below which substantially no hydrolysis takes place and electrolyzed in a cell of the diaphragm type at a sufliciently high current to precipitate the lead electrolytically.

The single figure of the drawing is a flow sheet of a cyclic process for the electrowinning of chromium embodying the lead-removing electrolyzing method of the invention.

The cyclic process for the electrowinning of chromium described in U. S. Patent 2,650,192 employs successive leaching, filtering, conditioning, crystallizing, second filtering, ageing, third filtering and dissolving steps prior to the introduction of the resultant purified solution into the electrolytic cell as cell feed. These steps are indicated as boxes 1, 2, 3, 4, 5, 10, 11, and 12, respectively, of the flow sheet. In addition, other purifying steps are accomplished through recycling procedures.

It has been found that the dissolved chrome alum crystals obtained in such cyclic process can be successfully treated in accordance with the invention, before introduction into the chromium-producing electrolytic cell as cell feed, to accomplish the substantial removal of virtually all of the contained lead impurity.

Referring specifically to the drawing, chrome alum crystals are dissolved in water and treated to adjust the pH of the solution to a value between approximately 1.0 and 2.25, at which substantially no hydrolysis takes place,

2,849,354 Patented Aug. 26, 1958 the solution, and the solution is introduced in the usual;

manner into the catholyte compartment of the electrolytic cell 13 as cell feed. This filtering step is indicated as box 12b of the flow sheet.

In the case of a chrome alum solution, having essentially the composition (NH -Cr (S0 -24H O, the unadjusted pH would be approximately 1.0 and adjustment in pH may be necessary to insure that the pH is within the desired range of 1.0 and 2.25. This pH adjustment may be accomplished by the addition of ammonia, barium hydroxide or any other base which will not interfere with subsequent plating of the chromium. With such solutions it has been found that the intensity of current and the length of the time of treatment for maximum precipitation of the lead impurity are approximately 60 ampere hours per gallon of cell feed solution, during which time the pH of the solution rises to approximately 2.25. To insure against hydrolysis, treatment of the solution with H anolyte from the chromium plating cells, or the like, may be required to maintain the pH of the solution during electrolyzing below the value of 2.25.

In an example of the method of the present invention, an electrolytic compartment cell of the diaphragm type was filled with cell feed solution and treated with 10,000 amperes for a total of 9 hours. The catholyte comprised 800 gallons of chrome alum solution. The anolyte com prised 200 gallons of the same solution. The following table sets forth the concentration of lead impurity in the catholyte and anolyte solutions at various times during electrolyzing.

Table Lead, g./l. Catholyte at start of test 0.0430 Catholyte after power was on for 3 hrs 0.0090 Catholyte after power was on for 5 hrs 0.0041 Catholyte after .power was on for 9 hrs 0.0045 Anolyte after power was on for 9 hrs 0.0046

As can be seen from the table above, the lead impurity concentration of the catholyte was reduced by more than 75 percent with approximately three hours of treatment and the maximum precipitation of lead, more than percent of the total originally contained, was obtained after five hours of electrolyzing. resented 62.5 ampere hours per gallon of catholyte and it was found that further treatment was not etfective in further lowering the lead impurity concentration of the solution. After such treatment, the precipitated lead was filtered from the solution through a plate and frame type filter press and the solution was fed into the electrolytic cell and electrolyzed forming at the cathode metallic chromium having less than 0.002 percent by weight of contained lead impurity.

While the lead-removal method of the present invention has been described hereinabove in connection with the cyclic process for the electrowinning of chromium described in U. S. Patent 2,650,192, it is to be understood that this method can be used with equal effectiveness in any cyclic process for the electrowinning of chromium employing a chromium-containing sulphate solution as cell feed.

What is claimed is:

1. In the electrowinning of chromium from chromium sulphate containing solutions, the improvement which comprises maintaining the pH of said solution at a value between approximately 1.0 and 2.25 to allow precipitation of the lead while retaining the chromium in solution,

This treatment repfirst electrolyzing said solution in at least one cell for greater than 30 ampere hours per gallon of solution to precipitate the contained-lead impurity, separating said precipitated contained-lead impurity from the resultant solution, and then electrolyzing the resultant solution in anothercell to electrowin vthe metallic chromium.

2. Method in-accordanc'e' with claim-1, wherein said solution is first electrolyzed for approximately 60 ampere'hours'per gallon of solution.

References Cited in the file of this patent UNITED STATES PATENTS Weaver May 20, 1941 Stockdale et a1 Mar. 3, 1942 Knox et al Nov. 2, 1943 Carosella et a1 Aug. 25, 1953 FOREIGN PATENTS Great Britain June 16, 1932 

1. IN THE ELECTROWINNING OF CHROMIUM FROM CHROMIUM SULPHATE CONTAINING SOLUTIONS, THE IMPROVEMENT WHICH COMPRISES MAINTAINING THE PH OF SAID SOLUTION AT A VALUE BETWEEN APPROXIMATELY 1.0 AND 2.25 TO ALLOW PRECIPITATION OF THE LEAD WHILE RETAINING THE CHROMUM IN SOLUTION, FIRST ELECTROLYZING SAID SOLUTION IN AT LEAST ONE CELL FOR GREATER THAN 30 AMPERE HOURS PER GALLON OF SOLUTION TO PRECIPITATE THE CONTAINED-LEAD IMPURITY, SEPARATING SAID PRECIPITATED CONTAINED-LEAD IMPURITY FROM THE RESULTANT SOLUTION, AND THEN ELECTROLYZING THE RESULTANT IN ANOTHER CELL TO ELECTROWIN THE METALLIC CHROMIUM. 